1. Technical Field
The description set forth herein relates generally to the field of visual displays and reusable display technology. More particularly, the description relates to a readdressable media, such as electric paper, including display members for use in elevated temperature applications.
2. Description of Related Art
Displays having rotatable elements provide numerous advantages over conventional electrically addressable visual displays, such as LCD and CRT displays. In particular, they are suitable for viewing in ambient light, they retain an image indefinitely in the absence of an applied electric field, and they can be made lightweight, flexible, foldable, and with many other familiar and useful characteristics of ordinary writing paper. Thus, at least in principle, they are suitable both for display applications and for so-called electric paper or interactive paper applications, in which they serve as an electrically addressable, reusable substitute for ordinary paper. For further advantages of such displays, see for example, U.S. Pat. No. 5,389,945, which is herein incorporated by reference.
Rotating element display material has also been disclosed in U.S. Pat. Nos. 4,126,854, 4,143,103 and 5,389,945, each herein incorporated by reference. The rotating element display material generally includes a substrate, a dielectric fluid and a set of rotatable elements. The rotating elements are embedded in an elastomer material which is then cured and swelled to create cavities around the elements.
The rotating elements, also referred to as bichromal spheres or bichromal balls, are optically and electronically anisotropic. For example, a rotating element has two distinct hemispheres, one black and one white, with each hemisphere having a distinct electrical characteristic giving it a dipole moment with respect to a dielectric fluid. An image is formed on the electric paper by applying an external electric field which rotates the bichromal sphere differentiated by the contrasting charges of the two hemispheres of the rotating element. The difference in charges between the hemispheres of the ball causes the ball to act like a dipole in the presence of an electrical field, which in turn causes the ball to rotate until its dipole vector lines up in the direction of the externally applied electric field. The image may stay in place until a new voltage pattern is applied, which erases the previous image and generates a new one.
Current multichromal display devices are often produced by the “swollen sheet” method. In this method, bare multichromal beads, randomly mixed and dispersed in a silicone elastomeric sheet, are rendered rotatable by swelling the elastomer in silicone oil. Pockets of oil form around each bead, and the beads detach from the elastomer-bead interface. The resulting device thus includes a swollen elastomeric coating of bichromal particles. Additional details about the swollen sheet production method may be found in, for example, U.S. Pat. No. 6,441,946, which is incorporated herein by reference in its entirety.
Numerous patents describe rotational elements, their manufacture, incorporation in display systems or substrates, and related uses and applications. Exemplary patents include, but are not limited to: U.S. Pat. Nos. 5,262,098; 5,344,594; 5,604,027 reissued as Re 37,085; U.S. Pat. Nos. 5,708,525; 5,717,514; 5,739,801; 5,754,332; 5,815,306; 5,900,192; 5,976,428; 6,054,071; 5,989,629; 6,235,395; 6,419,982; 6,445,490; and 6,703,074, all of which are hereby incorporated by reference.
Display members are commonly used for indoor electronic signage applications. Outdoor applications are limited because the base polymer used in media is a nonfunctional polyethylene, such as POLYWAX® 1000 or POLYWAX® 2000 from Baker Petrolite Corporation, Sugarland, Tex. The reason for the limited application is that at higher temperatures, it is believed that a fraction of the POLYWAX® leaches out to silicone fluid typically utilized inside the device, such as at about 50° C. to about 60° C. These unwanted materials are then carried or dispersed in the silicone fluid, thereby hindering bead rotation. As a result, the optical performances of the display members or devices are significantly reduced. At present, the application temperature must generally be maintained below about 40° C. by various cooling methods, which in turn are costly and there is no successful method to prevent the noted leaching difficulties described generally above.
Accordingly, there is a need for display members and related devices having improved optical performance at elevated temperatures.